Among all medical imaging techniques, there is an increasing trend to use magnetic resonance imaging (MRI) because of its non-ionized feature and high-resolution image quality. However, strong radio frequency (RF) fields generated by MRI systems can cause heating effects when patients are being scanned. This can be a significant issue when patients with metallic medical devices (e.g., external fixation devices) are scanned. Localized energy can be deposited near the tips of these medical devices, which may potentially lead to permanent tissue damage. This is particularly severe when patients with an external fixation device are being scanned for the fact that most external fixation devices are made up of nonmagnetic metal to maintain mechanical strength. When a typical external fixation device undergoes MRI, the metallic parts will interact strongly with the electromagnetic field and may produce induced electromagnetic energy inside human subjects. Furthermore, because only a small portion of metallic components are inside the human body, whereas the major portion of the device is outside, the highly condensed electromagnetic energy can only dissipated in a limited volume of tissue, which results in a very high increase in local temperature inside the human body. Luechinger et al. evaluated a group of large external fixation clamps and frames in MR environment and found a maximum of 9.9° C. increase in the local temperature at the tip of metallic pin inside the patients tissue. See, e.g., J. Biomed. Mater. Res. B, Appl. Biomater., 2007, 82:17-22.
To reduce the high-risk temperature increase, the method of using electrical (not thermal) absorption material to change the heating distribution was considered. Liu et al. studied the effect of electrical insulated layer material, and found it as a potential way to reduce the induced RF heating. See, e.g., Lie et al., Electromagnetic Biology and Medicine, 2014, Vol. 33, No. 3, Pages 223-227. However, the capability of insulated layer for RF heat reduction is limited, as the operating radio frequencies are relatively high (commonly above 64 MHz) and the induced current can still couple inside the human body.
The inventor unexpectedly found a solution that effectively reduces the RF heating (or RF-induced heating) in external fixation devices.